feat: record usage metadata on compaction events

[https-sam]

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Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Closes: [BUG]: Event Compaction event doesn't have token usage metadata. #4014
• Related: N/A

2. Or, if no issue exists, describe the change:

If applicable, please follow the issue templates to provide as much detail as
possible.

Problem:
Compaction runs a separate LLM call to summarize events, but the resulting
compaction event did not carry usage_metadata, so token usage for compaction
was lost in session analytics.

Solution:
Capture the summarizer LLM response usage_metadata and attach it to the
compaction Event so it is persisted and queryable like other LLM usage.

Testing Plan

Please describe the tests that you ran to verify your changes. This is required
for all PRs that are not small documentation or typo fixes.

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Please include a summary of passed pytest results.

• .venv/bin/python -m pytest tests/unittests/apps/test_llm_event_summarizer.py
• 5 passed

Manual End-to-End (E2E) Tests:

N/A

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

Additional context

As a potential enhancement, we could tag compaction events in custom_metadata like usage_kind=compaction so analytics can easily include/exclude summarization cost without inspecting actions.compaction.

[gemini-code-assist]

Summary of Changes

Hello @https-sam, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a gap in session analytics by ensuring that token usage from LLM calls made during event compaction is no longer lost. By capturing and attaching usage_metadata to compaction events, the system can now accurately track the costs associated with summarization processes, providing a more comprehensive view of LLM usage within user sessions.

Highlights

• Usage Metadata Capture: Introduced a mechanism to capture usage_metadata from LLM responses during the event compaction process.
• Event Enrichment: Attached the captured usage_metadata directly to the generated compaction Event objects, ensuring this data is persisted.
• Unit Testing: Added a new unit test (test_maybe_compact_events_includes_usage_metadata) to verify that usage_metadata is correctly propagated and stored with compacted events.

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[gemini-code-assist]

Code Review

This pull request successfully adds tracking for token usage during compaction events by capturing usage_metadata and attaching it to the resulting Event. The implementation is straightforward and includes a new unit test to verify the functionality. I've included one suggestion to make the logic for capturing the metadata more robust against potential edge cases.